tcp: annotate data-races around fastopenq.max_qlen

[ Upstream commit 70f360d ]

This field can be read locklessly.

Fixes: 1536e28 ("tcp: Add a TCP_FASTOPEN socket option to get a max backlog on its listner")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20230719212857.3943972-12-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

include/linux/tcp.h

@@ -458,7 +458,7 @@ static inline void fastopen_queue_tune(struct sock *sk, int backlog)
 	struct request_sock_queue *queue = &inet_csk(sk)->icsk_accept_queue;
 	int somaxconn = READ_ONCE(sock_net(sk)->core.sysctl_somaxconn);
 
-	queue->fastopenq.max_qlen = min_t(unsigned int, backlog, somaxconn);
+	WRITE_ONCE(queue->fastopenq.max_qlen, min_t(unsigned int, backlog, somaxconn));
 }
 
 static inline void tcp_move_syn(struct tcp_sock *tp,

net/ipv4/tcp.c

@@ -3623,7 +3623,7 @@ static int do_tcp_getsockopt(struct sock *sk, int level,
 		break;
 
 	case TCP_FASTOPEN:
-		val = icsk->icsk_accept_queue.fastopenq.max_qlen;
+		val = READ_ONCE(icsk->icsk_accept_queue.fastopenq.max_qlen);
 		break;
 
 	case TCP_FASTOPEN_CONNECT:

net/ipv4/tcp_fastopen.c

@@ -312,6 +312,7 @@ static struct sock *tcp_fastopen_create_child(struct sock *sk,
 static bool tcp_fastopen_queue_check(struct sock *sk)
 {
 	struct fastopen_queue *fastopenq;
+	int max_qlen;
 
 	/* Make sure the listener has enabled fastopen, and we don't
 	 * exceed the max # of pending TFO requests allowed before trying
@@ -324,10 +325,11 @@ static bool tcp_fastopen_queue_check(struct sock *sk)
 	 * temporarily vs a server not supporting Fast Open at all.
 	 */
 	fastopenq = &inet_csk(sk)->icsk_accept_queue.fastopenq;
-	if (fastopenq->max_qlen == 0)
+	max_qlen = READ_ONCE(fastopenq->max_qlen);
+	if (max_qlen == 0)
 		return false;
 
-	if (fastopenq->qlen >= fastopenq->max_qlen) {
+	if (fastopenq->qlen >= max_qlen) {
 		struct request_sock *req1;
 		spin_lock(&fastopenq->lock);
 		req1 = fastopenq->rskq_rst_head;